2. Melt the 6 tablespoons of butter in a large saucepan over medium heat. Stir in mushrooms and cook until softened, about 5 minutes. Stir in flour until well blended. Slowly add chicken broth and milk, whisking all the time. Bring to a boil, reduce heat, and cook until sauce is thickened and smooth, about 5 minutes. Mix in chicken and rice. Pour into prepared dish.

3. Mix together the breadcrumbs, Parmesan, and melted butter. Sprinkle on top of rice mixture.

4 Bake until the sauce is bubbling and topping is golden brown, 25-35 minutes.